An integrated circuit (IC) may include millions of circuit elements that can be on or off at any instant in time. The voltage level in the IC tends to rise or fall according to the load (the number of active elements on or off) at any given time. Moreover, when a large number of elements transition between on or off at the same instant, the voltage level can change dramatically at that instant in time. This dramatic voltage change is often called a transient change because the voltage level droops downward or spikes upward momentarily and then eventually settles to a steady-state value.
In order for a circuit element to transition between on and off, the element is likely to require a certain minimum voltage level. In other words, if the voltage level drops too low, the circuit element may not reliably transition, which can cause unacceptable errors. One approach to this problem, at least for steady-state voltage changes, is a guard voltage.
A guard voltage guards against the voltage level dropping too low. That is, the source voltage is set high enough so that the voltage level remains higher than the minimum voltage needed even during times of heavy load. The trade-offs, however, for a guard voltage are power consumption and heat generation. Millions of circuit elements in an IC can consume a great deal of power and generate a great deal of heat. The power consumed, and the heat generated, are often directly related to the voltage level at which the elements operate. A guard voltage that is high enough to guard against heavy loads is likely to be much too high to be practical in terms of power consumption and heat generation in many modern ICs.
Furthermore, transient voltage changes may simply be too big to guard against. One approach for dealing with transient changes is to require a settling time following a transition. That is, transitions can be scheduled far enough apart in time so that the voltage level has time to settle to a steady state between transitions. Of course, the trade-off for a transient settling time is speed, potentially limiting the clock rate or operating frequency of an IC.
Another approach to dealing with changes in voltage levels is a voltage regulator. A voltage regulator can adjust a source voltage to try to compensate for changes detected in the voltage level at a load. There is virtually always some delay and inaccuracy involved, so a voltage regulator is often used in combination with a guard voltage and a settling time. Depending on the accuracy and responsiveness of a particular voltage regulator, the guard voltage and settling time may be larger or smaller. In other words, a better voltage regulator can support smaller guard voltages and smaller settling times, and, consequently, a better voltage regulator can provide faster, cooler, and more power-efficient ICs.